Method of heat-treating steel machine parts



United States Patent 3,357,869 METHOD OF HEAT-TREATING STEEL MACHINE PARTS Konstantin Zakharovich Shepeljakovsky, Moscow, USSR, assignor to Moskovsky Automobilny Zavod I. A. Likhachev, Moscow, U.S.S.R. No Drawing. Filed Jan. 29, 1965, Ser. No. 430,242 2 Claims. (Cl. 148-165) The present invention relates to the heat-treatment of steel, and more particularly to the hardening of compli cated steel machine parts insuring high-strength properties.

It is known that in the industrial induction surface hardening, a steel of lower hardenability is used, which increases the fatigue strength of machine parts having reinforcing elements with thin cross-sections. However, in the course of a long service of such parts, the so-called contact breaking off occurs in the surface layer, which is due to the fact that the contact strength of parts is insufliciently high on account of a relatively low (from 0.4 to 0.7 percent) carbon content in the surface layer.

Though attempts have been made to eliminate said difficulties and drawbacks, such attempts have not proven successful.

An object of the present invention is to increase the contact strength and wear resistance of machine parts by providing an advantageous combination of strength properties with a high fatigue strength.

This object is achieved by supplying into a zone of heating for hardening such substances that make it possible to carry out the processes of nitride hardening simultaneously with surface hardening.

While the known method of surface hardening allowed to obtain two zones of hardness in machine parts, i.e., a layer of surface hardening up to 2 mm. deep with the hardness number of 60 to 64 R and a hardened core with the hardness number of 30 to 48 R the proposed method allows of obtaining three zones of hardness.

This is achieved by subjecting machine parts made of a lower hardenability carbon steel, i.e., from a steel having the radius of the semi-martensitic zone (transition region consisting of equal parts of martensite and troostite) from 6 to mm. and containing about 0.4 to 0.7 percent carbon, to induction heating for hardening with the simultaneous supply into the heating zone of carbonand nitrogen-containing substances in a gaseous, liquid and solid state, more particularly a mixture of ammonia with the methane, kerosene, synthol or solid pastes with cyano-containing salts.

Said materials are supplied into the heating zone in the following way: gaseous substances are pressure supplied from a pipe-line or a tank; liquid substances are supplied by means of a dropper or a plunger atomizer; solid substances are applied directly onto the part being treated.

When heating by induction, parts are to be held at a temperature below the grain growth level, and consequently below the hardenability increase level, for example, at 830 to 850 C. On completion of the heating, parts are cooled at a fast rate, for example, by means of a water shower.

The described heat-treatment results in the surface hardening of parts to a depth, which is determined by the depth of hardenability of a steel employed, usually from 1 to 2 mm., and the saturation of the surface layer with carbon and nitrogen.

3,357,869 Patented Dec. 12, 1967 Thus, hardened zones of three kinds develop in parts, such as: a thin surface layer of nitride hardening 0.02 to 0.1 mm., having the hardness number of 67 to 70 R (1000 to 1200 H.,); a subsequent layer of surface hardening 1 to 2 mm. deep, having the hardness number of 58 to 62 R and a hardened core, having the hardness number of 30 to 35 R The thin surface layer, as obtained by the nitride hardening, together with the martensite layer of the surface hardening insure a high contact strength protecting the part against the wear and the contact breaking off. The surface-hardened layer together with the hardened core provide for the high static and fatigue strength. The presence of a hardening which is not completely through the part sets up internal compressive stresses, thus increasing the fatigue strength.

For a better understanding of the present invention, the following example is given by way of illustration.

Gears of the modulus equal to 4.23 m., 178 mm. in diameter, having the axial depth of teeth equal to 25 mm., are manufactured of a lower hardenability steel with a carbon content of 0.4 to'0.45 percent and critical semimartensite diameter of 6 to 9 mm.

The gears are subjected to induction heating for hardening at a temperature of 850 C. with the isothermal holding for seconds.

Simultaneously, a mixture of the ammonia with methane is supplied into the heating zone.

On completion of heating, the gears are quenched by means of a water shower.

Said treatment results in the formation on the surface of a part of a layer of nitride hardening 0.05 to 0.1 mm. deep having the hardness number of 67 to 68 R (1000 to 1100 H a layer of surface hardening 1.2 to 1.8 mm. deep having the hardness number of 60 to 62 R and a hardened core having the hardness number of 30 to 35 R The optimum combination of the hardness of the surface layer and of the impact strength of the core makes it possible to manufacture parts of complicated shape, the strength characteristics of which exceed those of the parts hardened by means of the methods known hitherto.

The application of the present invention will provide for cheap and effective manufacturing of machine parts with desirable optimum mechanical properties. Possible modifications and variations that may be made without departing from the spirit of the invention should be regarded as falling within the scope of the invention if defined by the appended claims.

Having thus described the invention, I claim:

1. Method of heat-treating machine parts comprising manufacturing said parts from a lower hardenability steel, induction heating said parts being treated for hardening to a temperature of about 850 C. with a simultaneous supply into the zone of induction heating of substances containing carbon and nitrogen selected from the group of ammonia, hydrocarbons, cyanide salts, and cyano containing compounds, a subsequent holding of said parts during 30 to seconds at a temperature of about 830 to 850 C. followed by quenching said parts with the water shower such that each such part has three hardened zones: a carbonitride hardened zone 0.020.1 mm. deep, HR =6770 (H =1,0001,200), a martensitehardened zone with HR =5862 and a troostite and sorbite core with HR =3035.

2. Method of heat-treating machine parts comprising manufacturing said parts from a lower hardenability steel,

References Cited UNITED STATES PATENTS Kinzel 14816.6 Cowan 14816.5

Holt 14816.5

Unterweiser 14816.6 X Mitchell et al. 148-166 X DAVID L, RECK, Primary Examiner.

0 H. F. SAITO, Assistant Examiner. 

1. METHOD OF HEAT-TREATING MACHINE PARTS COMPRISING MANUFACTURING SAID PARTS FROM A LOWER HARDENABILITY STEEL, INDUCTION HEATING, SAID PARTS BEING TREATED FOR HARDENING TO A TEMPERATURE OF ABOUT 850*C. WITH A SIMULTANEOUS SUPPLY INTO THE ZONE OF INDUCTION HEATING OF SUBSTANCES CONTAINING CARBON AND NITROGEN SELECTED FROM THE GROUP OF AMMONIA, HYDROCARBONS, CYANIDE SALTS, AND CYANO CONTAINING COMPOUNDS, A SUBSEQUENT HOLDING OF SAID PARTS DURING 30 TO 100 SECONDS AT A TEMPERATURE OF ABOUT 830 TO 850*C. FOLLOWED BY QUENCHING SAID PARTS WITH THE WATER SHOWER SUCH THAT EACH SUCH PART HAS THREE HARDENED ZONES; A CARBONTRIDE HARDENED ZONE 0.02-0.1 MM. DEEP, HRC=67-70 (HV=1,000-1,200), A MARTENSITEHARDENED ZONE WITH HRC=58-62 AND A TROOSTITE AND SORBITE CORE WITH HRC=30-35. 